1. Field of the Invention
This invention relates to the field of video inspection systems for container rims, and more particularly to an illumination device for bottle sealing rims, the device directing light inwardly and obliquely of the rim for developing a video image with minimum brightness variation apart from variations due to defects in the rim.
2. Prior Art
The prior art includes the use of techniques relying on light scattering by rim defects for discriminating between good rims and bad rims in containers such as soft drink bottles and the like. A video scanning system in which digitized data representing pixel brightness is compared for adjacent pixels to discriminate defects is disclosed, for example, in U.S. Pat. No. 4,454,542--Miyazawa, the video data processing aspects of which are incorporated herein by reference. Brightness comparison techniques rely on even illumination of the article being inspected, because uneven illumination may cause brightness variations, reflections and the like that are not easily distinguished from defects. The Miyazawa system is effective from an image-analysis standpoint, but teaches illuminating the container and its rim by light from an annular source above the container. The light is directed substantially along the axis of the container, which is generally-cylindrical.
Axially illuminating a container can be arranged by directing light through a transparent base while viewing axially toward the rim, or by means of an annular diffuse light source through which annular source the rim is viewed from above. In the event the containers are to be movable relative to the inspection system without undue mechanical complexity, at right angles to the container axis, such an annular source is necessarily positioned at a space above the rim, and accordingly cannot provide a diffuse illumination of the rim from the sides of the conveying path, in particular including obliquely-directed light rays passing through the rim upwardly and inwardly with respect to the axis of view. On the other hand, only an annular shape is symmetrical and will evenly illuminate the rim around its circumference.
A typical bottle rim for a soft drink or the like has a rounded annular or toroidal sealing surface defining a shape that can sealably receive a cap and can be placed against a customer's lips. For some bottles the rim is threaded, for others merely rounded. Almost any defect in a rim is grounds to reject the container. However, defects that occur in threads or around the lower part of the toroidal sealing surface are not readily visible in a view along the bottle axis, and may even be concealed by too much reliance on axially-oriented illumination. Rim inspection systems therefore are characterized either by asymmetrical illumination or illumination that is not directed obliquely upward to disclose defects present in the area of a lower edge of the sealing surface.
U.S. Pat. No. 3,631,255--Gender, et al has means reaching over the rim. The patent teaches a rim inspection system in which light is to be directed inwardly along a conveying path defining an arc by means of a plurality of point sources spaced evenly along the conveying path. This patent teaches illuminating the rim using substantially-radially directed light. Any defect such as a sealing ring chip or irregularity causes additional light to be directed upwardly along the axis of the container due to scattering of light beams. The light level is detected by a photo multiplier detector and presumably compared to a rejection threshold. The patent requires developing a sharp variation in the level of axially-directed light between a good rim in which radial light rays pass radially through the rim and a bad rim in which the radially-directed rays are re-directed along the axis of the container to the detector. This average light level technique is somewhat different than a video inspection system in which results are best if the rim is evenly illuminated and the brightness level of adjacent pixels is compared. The average level technique does not require even illumination as does the video technique.
U.S Pat. Nos. 4,391,373--Wiggins and 4,213,042--Beach, et al, also teach devices having channel-like housings extending over the rim. These patents both concern paired light sources and photocells. Although the optical paths between these paired elements are inclined obliquely to the axis of the bottle, the disclosure does not suggest how such an arrangement might be adapted to a video analysis technique in which diffuse, even illumination is needed. Their disclosures are much like Gender's threshold comparison, but use discrete optical paths.
U.S. Pat. Nos. 4,208,130--Saconney, et al and 3,386,579--Schulze teach devices in which an effort is made to illuminate a rim such that light is directed at a plurality of angles toward or away from the rim. These patents show a means for expanding the photocell pair or plural light source ideas of the prior art in a way that more evenly illuminates the rim, but these patents, together with those mentioned above are characterized by a plurality of discrete sources that would produce light variations of themselves, apart from defects. The patents lack an illumination element in which a bottle rim is evenly diffusely illuminated for video analysis, including oblique upward illumination, but allowing the bottles to move relative to the inspection device without interruption.
According to the invention, a diffuse light-source element defined by a number of strobe lamps or the like and a monolithic, internally-reflective prism device having diffuse side walls directing light radially inwardly and obliquely upward adjacent the rim, is provided together with a pixel-comparison video analysis system viewing the container along its axis, the video view being taken through an opening in the monolithic illumination element. The diffuse illumination over a wide area provides a very even illumination for the bottle. Masking of certain parts of the opening through which the video detector views allows the element to provide some illumination even toward the sides directed fore and aft along the conveying path. Preferably, the illumination element and its prism parts are substantially longer in the conveying direction than the bottle, thereby minimizing directional effects, and illuminating the rim substantially evenly around its circumference and from oblique aspects above and below.
Reflective prism elements according to the invention are trapezoidal in section, having an outer internally-reflective wall disposed at about 45.degree. to bottle axis and to the light source, directing light rays inwardly toward the rim. A diffuse surface adjacent the rim defines a wall of a channel enclosing over the rim oriented substantially parallel to the bottle axis. The diffuse surface receives light internally reflected from the source and internally reflective prism, illuminating the rim in the area of the viewing port. Preferably, two such prism elements are provided on opposite sides of the conveying path, and are affixed via a light transmissive panel having the viewing port therein, and to which the light sources, preferably strobe flash tubes operable to "freeze" the image in place, are attached. In this manner, high levels of illumination are provided and the illumination is quite even and includes oblique rays. The illumination is adequate for high resolution comparisons of pixel brightness, facilitating discrimination of rim defects without the need to stop the bottles in their movement along the conveying path.